Implications for Thrust-Related Shortening Punctuated by Extension From P-T Paths and Geochronology of Garnet-Bearing Schists, Southern (Çine) Menderes Massif, SW Turkey

Abstract

The Menderes Massif, Turkey, is a type locality for deciphering the plate tectonic response from collision- to extension-driven exhumation. Conventional thermobarometry and garnet pressure-temperature (P-T) paths from isochemical phase diagrams were calculated across a major fault (Selimiye Shear Zone, SSZ) bounding the southern edge of the Menderes Massif. Both approaches yield similar garnet rim temperatures (from 555 to 671 °C), but estimated P differs by between 8 and 15 kbar. Three garnets north of the SSZ reveal N-shaped P-T paths, whereas paths from three samples south of the SSZ show a simple increase in P-T. Monazite and zircon were dated in thin section from the same rocks using Secondary Ion Mass Spectrometry and Laser Ablation Inductively Coupled Plasma-Mass Spectrometry, respectively. Textural relationships of monazite within garnet appears indicative of post-garnet growth. The amount of monazite common 204Pb and 137Ba+/Th+ significantly exceeds what is observed for the monazite age standard, suggesting their ages mark fluid-driven events, loosely constrained to Late Eocene-Early Miocene. Some zircon ages are consistent with Cambro-Ordovician ages reported elsewhere in the region, and other ages are Neoproterozoic and Permian-Triassic, a period not previously recognized in this area. Despite the lack of age constraints for the duration of garnet growth, we present a thermal model to understand the meaning of the N-shaped path. These paths are best reproduced by thermal models incorporating SSZ thrusting before and after denudation. This paper presents an example of the insight from high-resolution P-T paths, and an example of denudation within a prograde metamorphic event.